I don't think you "should" use anything. There's plenty of x86/ARM/MIPS projets out there. I applaud the fact that they went with something different.

Isn't building what you consider fun what hacking is all about? I think it's healthy to completely ignore "usefulness" at times and doodle along :)

Also, if we're talking about theoretical "students," quality is often more important than subject. Academics often recommend undergraduates "choose your course by the teacher, not the subject." Same principle here, if they work on something fun and they have fun, they will learn more.

Or http://pdos.csail.mit.edu/6.828/2014/xv6.html -- the os used in a similar course at mit (based on Unix v6, as in ye olde Lions book, but updated to use modern C and modern hardware).

x86 assembly, however, can also lead to existential crises. 'I have to do WHAT? And they did it this way because of a decision made when designing the 286? Why am I in this field again?'